The whitefish of Europe's Alpine lakes were once a single species, but after the Ice Age they split into separate species, adapting their look and lifestyle to their particular watery home. But this amazing biodiversity would have to be sacrificed.

We often think of a species in terms of its ability to reproduce - if one individual can breed with another, then they are both part of the same species. And while this genetic compatibility is a good rule of thumb, it's not entirely the case, as lots of closely related species can indeed crossbreed and, if this happens on a large enough scale, even form hybrid species. Fish species that split apart as much as 20 million years ago can still hybridize back together again, effectively rendering their time as separate species an evolutionary cul-de-sac.

It's thought that as much as 88% of fish species - not to mention 55% of mammals - could still successfully hybridize with at least one other species, although a lot of these fish don't actually live near to their prospective merging partner, which makes the risk relatively low. These Alpine whitefish, however, are right on top of each other. As the last century's pollution caused oxygen levels in the lakes to crash, the fish were forced the different species to merge together in the hopes of survival. Broadly speaking, this has worked - the question though is how many of the original fish species really still exist. New Scientist explains the discoveries of Swiss researcher Ole Seehausen:

Seehausen and colleagues now think that this oxygen crash forced species to merge. They studied the whitefish populations in 17 lakes, each of which was also studied in the 1920s, before the eutrophication began. Back then, the deeper lakes had more species. The number of species in the lakes has fallen 38 per cent since the 1920s, and the remaining species have become more similar in shape. What's more, lakes that have experienced more eutrophication have fewer species. Seehausen found that the remaining species sometimes carry genetic markers previously found only in extinct species, suggesting that those species have hybridised themselves out of existence.

The hybridization process is particularly troublesome because it can often hide the true extent of environmental carnage. The merger of species can keep population levels relatively stable, meaning we don't even notice individual species are disappearing until it's too late.